Method for fabricating transistor with thinned channel

ABSTRACT

A method of fabricating a MOS transistor having a thinned channel region is described. The channel region is etched following removal of a dummy gate. The source and drain regions have relatively low resistance with the process.

FIELD OF THE INVENTION

The invention relates to the field of semiconductor processing fortransistors having thin channel regions.

PRIOR ART AND RELATED ART

The trend in the fabrication of complementary metal-oxide-semiconductor(CMOS) transistors is to have small channel regions. Examples of atransistor having a reduced body which includes the channel region alongwith a tri-gate structure are shown in US 2004/0036127. Other smallchannel transistors are delta-doped transistors formed in lightly dopedor undoped epitaxial layers grown on a heavily doped substrate. See, forinstance, “Metal Gate Transistor with Epitaxial Source and DrainRegions,” application Ser. No. 10/955,669, filed Sep. 29, 2004, assignedto the assignee of the present application.

One problem with some of these devices is the generally high externalresistance that comes about from the thinning of the source and drainregions, sometimes at the edges of the gates. Other devices have similarproblems that result in higher external resistance, such as limitedavailable cross-sectional area for source and drain regions. Theseproblems are discussed in conjunction with FIGS. 1A and 1B.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plane view of a prior art transistor taken with a scanningelectron microscope.

FIG. 1B is a cross-sectional, elevation view of a depletion modetransistor fabricated on a heavily doped substrate. This figure wastaken from the patent application cited in the Prior Art and Related Artsection of this application.

FIG. 2 is a cross-sectional, elevation view of a silicon-on-insulator(SOI) substrate.

FIG. 3 is a perspective view of the structure of FIG. 2, after theformation of a silicon body, sometimes referred to as a fin.

FIG. 4 illustrates the structure of FIG. 3, after a dummy gate isfabricated and during a first ion implantation process.

FIG. 5 illustrates the structure of FIG. 4, after spacers are fabricatedand during a second ion implantation step.

FIG. 6 illustrates the structure of FIG. 5, after forming a dielectriclayer.

FIG. 7 illustrates the structure of FIG. 6, after removal of the dummygate.

FIG. 8 is a cross-sectional, elevation view of the structure of FIG. 7taken through section line 8-8 of FIG. 7.

FIG. 9 illustrates the structure of FIG. 8, after an etching step whichthins the channel region.

FIG. 10 illustrates the structure of FIG. 9, after forming a high-k gateinsulating layer and a metal gate layer.

FIG. 11 illustrates the structure of FIG. 10 after planarization.

FIG. 12, which illustrates alternate processing, is a cross-sectional,elevation view, at a point in the processing similar to FIG. 8. In thisalternate processing, a hard mask, used to define the silicon body,remains atop the channel region.

FIG. 13 illustrates the structure of FIG. 12, as viewed through thesection lines 13-13 of FIG. 12.

FIG. 14 illustrates the structure of FIG. 13, after etching whichreduced the width of the body in the channel region.

FIG. 15 is a cross-sectional, elevation view of a SOI substrate.

FIG. 16 illustrates the structure of FIG. 15, after etching of thesilicon layer.

FIG. 17 illustrates the structure of FIG. 16, with epitaxially grownsource and drain regions.

FIG. 18 illustrates the structure of FIG. 17, after a dielectric layeris formed around the structure and a gate region is exposed.

FIG. 19 illustrates the structure of FIG. 18, after additional etching.

FIG. 20 illustrates the structure of FIG. 19, after etching, which thinsthe channel region.

FIG. 21 illustrates the structure of FIG. 20, after forming aninsulating, high-k layer, and a metal gate layer.

FIG. 22 illustrates the structure of FIG. 21, after planarization.

FIG. 23 is a block diagram, incorporating the above-illustrated thinnedchannel transistors in a system.

DETAILED DESCRIPTION

A process for fabricating CMOS field-effect transistors and theresultant transistors are described. In the following description,numerous specific details are set forth, such as specific dimensions andchemical regimes, in order to provide a thorough understanding of thepresent invention. It will be apparent to one skilled in the art thatthe present invention may be practiced without these specific details.In other instances, well-known processing steps, such as cleaning steps,are not described in detail, in order to not unnecessarily obscure thepresent invention.

A problem associated with small body devices is illustrated in FIG. 1A.A tri-gate structure 10 is shown traversing a silicon body at thechannel region 14 of a transistor. The semiconductor body or fin hasbeen thinned at the gate edges 11. This thinning is the result ofprocessing used for defining the body, forming spacers, and cleaning ofoxides. This processing can so reduce the body such that it may nolonger have sufficient silicon seed to support the growth of anepitaxial layer. Often, as much as 20-50% of the body at the edge of thegate can be lost during such processing. In addition to yield loss, thisresults in higher source/drain resistance and the consequentialreduction in transistor performance.

A similar problem is shown in FIG. 1B, where the n type or intrinsicchannel region 15 is formed between the relatively thin regions 16 ofthe source and drain extension regions of a depletion mode planartransistor. Thus, the problem of thinning at the gate edges is notlimited to tri-gate structures, or for that matter, SOI substrates, butcan also occur in a bulk silicon layer or a delta-doped transistor, asshown in FIG. 1B. In the structure of FIG. 1B, an epitaxial layer isgrown on a heavily doped substrate 14. An etchant discriminates betweenthe epitaxially grown layer and the substrate 14, allowing the channelregion 15 to be defined. The source and drain regions 17 are grownfollowing the etching of the region 15. The structure is described inthe application referenced in the Prior Art and Related Art section ofthis application.

In a first embodiment, transistors are fabricated on an oxide layer 20which is disposed on a silicon substrate 21 shown in FIG. 2. Thetransistor bodies are fabricated from a monocrystalline, silicon layer24 disposed on the oxide layer 20. This SOI substrate is well-known inthe semiconductor industry. By way of example, the SOI substrate isfabricated by bonding the oxide layer 20 and silicon layer 24 onto thesubstrate 21, and then planarizing the layer 24 so that it is relativelythin. Other techniques are known for forming an SOI substrate including,for instance, the implantation of oxygen into the silicon substrate toform a buried oxide layer. Other semiconductor materials, other thansilicon, may also be used such as gallium arsenide.

The layer 24 may be selectively ion implanted with a p type dopant inregions where n channel transistors are to be fabricated, and with a ntype dopant in those regions where p channel devices are to befabricated. This is used to provide the relatively light dopingtypically found in the channel regions of MOS devices fabricated in aCMOS integrated circuit.

In the description below, for the first embodiment, the fabrication of asingle n channel transistor is described. As will be appreciated in thetypical integrated circuit, both n and p channel devices are fabricated.Also, in the processing for the first embodiment, a protective oxide(not shown) is disposed on the silicon layer 24 followed by thedeposition of a silicon nitride layer. The nitride layer acts as a hardmask to define silicon bodies such as the silicon body 25 of FIG. 3.

Assume for a particular process that the silicon body, in the channelregion of a field-effect transistor, should ideally have a height of 20nm and a width of 20 nm. Using the prior art processing associated withthe transistor of FIG. 1, the thickness of the silicon layer from whichthe body is etched would also have a thickness of 20 nm. As will be seenfor the embodiment of FIGS. 2-11, the layer 24 may initially be thickerthan 20 nm, and will subsequently be thinned in the channel region. Thisthinning only occurs in the channel region, leaving the source and drainregions thicker, thereby reducing the external resistance. This willbecome more apparent in the description below.

A polysilicon layer is formed over the structure of FIG. 3 and etched todefine a dummy gate 30 which extends over the body 25 as seen in FIG. 4.(A dummy gate oxide which subsequently acts as an etch stop is notshown.) The region of the body 25 below the dummy gate 30, as will beseen, is the channel region for this replacement gate process. Once thedummy gate 30 has been defined, phosphorous or arsenic may be implantedinto the body 25 in alignment with the dummy gate, as illustrated by theion implantation 26. This ion implantation defines the tip or extensionsource and drain regions frequently used in CMOS transistors.

Next, a layer of silicon nitride is conformally deposited over thestructure of FIG. 4, and is used to fabricate the spacers 38 shown inFIG. 5. Ordinary, well-known, anisotropic etching may be used tofabricate the spacers. In one embodiment, a carbon-doped nitride, dopedwith 5-13% carbon concentration is used for the spacers. As will bediscussed later, prior to the formation of the nitride layer, an oxidelayer present on the body 25 is removed. This cleaning process is one ofthe processes that typically reduces the thickness of the body at theedges of the gate. After the spacer formation, the main part of thesource and drain regions are formed through ion implantation 35 shown inFIG. 5. For the n channel device, arsenic or phosphorous is used with animplant dose of up to 1×10¹⁹-1×10²⁰ atoms/cm³.

A dielectric layer 40 is now conformally deposited over the structure ofFIG. 5, as shown in FIG. 6. This may comprise a silicon dioxide layerwhich will become an interlayer dielectric (ILD) in an integratedcircuit. A low-k dielectric or a sacrificial dielectric layer may beused. In any event, the layer 40 typically has the mechanical strengthto withstand a planarization process such as chemical mechanicalpolishing (CMP).

After the deposition and planarization of the dielectric layer 40, a wetetch is used to remove the dummy polysilicon gate 30, leaving theopening 45, as shown in FIG. 7. (The dummy gate oxide (not shown) isalso removed.) The cross-sectional view of FIG. 8 taken through sectionline 8-8 of FIG. 7, better shows the opening 45. Also better shown inFIG. 8, is that the spacers 38 are recessed at 47 into the body 25. Asmentioned earlier, after the source and drain tip implant and before thespacer material is deposited, an oxide layer is removed from the body25. This results in the loss of some of the body material and, in part,accounts for the thinning occurring in the prior art structure shown at11 of FIG. 1A. The removal of this oxide is important in some processes,as will be discussed later.

Following the removal of the dummy gate, any oxide over the body 25within the opening 45 is removed in an ordinary cleaning step. Then, thestructure of FIG. 8 is placed in a selective silicon bath such as NH₄OHwhich reduces the size of the body within the opening 45. This etchingreduces both the height of the body as seen at 50 of FIG. 9, as well asthe width of the body. This allows the channel region of the body 25 tobe thinned to a target height and thickness. Again, assume that thetarget height and thickness of the body 25 in the channel region is20×20 nm. A thicker and wider body 25 may be initially formed since itis thinned in this etching step. Importantly, this etching step does notthin the body 25 outside of the channel region. Consequently, if thebody 25 is initially thicker and wider, the source and drain regionsremain thicker or wider after the channel region has been thinned. Thus,by starting with a thicker and wider body, there is more silicon leftafter the cleaning process, and the severe thinning shown at 11 of FIG.1A is avoided.

Next, a gate dielectric 60 is formed on exposed surfaces which includesthe sides and top of the body 25 lying within the opening 45. The layer60 also deposits on the interior sidewalls of the spacers 38 and on theupper surface of the dielectric layer 40. The gate dielectric, in oneembodiment, has a high dielectric constant (k), such as a metal oxidedielectric, for instance, HfO₂ or ZrO₂ or other high k dielectrics, suchas PZT or BST. The gate dielectric may be formed by any well-knowntechnique such as atomic layer deposition (ALD) or chemical vapordeposition (CVD). Alternately, the gate dielectric may be a growndielectric. For instance, the gate dielectric 60, may be a silicondioxide film grown with a wet or dry oxidation process to a thicknessbetween 5-50 Å.

Following this, also as seen in FIG. 10, a gate electrode (metal) layer61 is formed over the gate dielectric layer 60. The gate electrode layer61 may be formed by blanket deposition of a suitable gate electrodematerial. In one embodiment, a gate electrode material comprises a metalfilm such as tungsten, tantalum, titanium and/or nitrides and alloysthereof. For the n channel transistors, a work function in the range of3.9 to 4.6 eV may be used. For the p channel transistors, a workfunction of 4.6 to 5.2 eV may be used. Accordingly, for substrates withboth n channel and p channel transistors, two separate metal depositionprocesses may need to be used.

The metal layer 61 is planarized using, for example CMP, and theplanarization continues until at least the upper surface of thedielectric layer 40 is exposed, as shown in FIG. 11.

Standard processing is now used to complete the transistor of FIG. 11.

Alternate processing is next described in conjunction with FIGS. 12-14.The numbering used in FIG. 12 includes a “0” after the numbers used inFIGS. 2-11 for corresponding layers and members. For instance, thespacers 38 of FIG. 11 are shown as spacers 380 in FIG. 12. In FIG. 12, asubstrate 210, insulator 200 and body 250 are seen along with the sourceand drain regions.

The processing leading up to FIG. 12 is the same as the processing as inthe prior embodiment, with one exception. This difference is that thehard mask defining the bodies or fins, such as body 250, is notimmediately removed after defining the bodies. Rather, the polysilicondummy gate structure is formed over the hard mask. Then, where the bodyis not protected by the polysilicon dummy gate, the hard mask isremoved. Consequently, the hard mask 260 remains over the channelregion, and after the spacers are formed and dummy gate removed, thehard mask 260 is exposed in opening 450 of FIG. 12.

The structure of FIG. 12 is again illustrated in FIG. 13 from the viewtaken through the lines 13-13 of FIG. 12. In this view, the silicon body250 and mask 260 are shown with the spacer 380 in the background. Thedimension 280 of FIG. 13 is the width of the body 250 in the channelregion.

Wet etching is now used to etch the silicon body 250 with, for example,NH₄OH. This thins the width of the silicon body without changing itsheight, as shown in FIG. 14. Note, in FIG. 14 the remaining body 250 isnarrower than its original dimension 280, while its height has notchanged. For some semiconductor processes, better control may beobtained when only the width of the silicon body is etched. Thus, onlythe width of the body in the channel region is reduced to apredetermined target dimension, without reducing the height or width ofthe source and drain regions as this occurs.

After this etching step, the hard mask 260 is removed, then the high kdielectric and metal gates are formed, as was the case in the previousembodiment.

As mentioned earlier, the silicon dioxide layer, which typically ispresent on the silicon body, is removed before the deposition of thespacer material. This was discussed in conjunction with the recess 47 ofFIG. 8. This is done because an undesirable reaction may occur betweenthe oxide layer and the high k dielectric, if a high temperature annealis used to activate the doping in the source and drain regions after thereplacement gate is formed. At least the sides of the oxide layer, ifnot removed, may contact the high k dielectric and cause this problem.This is not a problem, however, if the annealing of the source and drainregions occurs before the high k dielectric is formed.

The thinning of the channel region described above can also be used on aplanar, bulk transistor or a transistor formed in a delta-dopedsubstrate. FIGS. 15-21 below describe the formation of a depletion modetransistor with raised source and drain regions, where controlledthinning of the channel region occurs.

FIG. 15 illustrates an SOI substrate having a base 100, oxide layer 102and a monocrystalline silicon layer 103. For the depletion modetransistor, the layer 103, or at least the region where the depletionmode transistor is fabricated, is lightly doped with an n type dopant oris intrinsic silicon, or other semiconductor material. A gate structureis fabricated on a dummy gate oxide layer 110. This structure comprisesa polysilicon dummy gate 104 and spacers 105.

Following this, as shown in FIG. 16, the semiconductor layer 103 isetched isotropically in alignment with the gate structure. This etchingundercuts the gate structure as illustrated at undercut 114. Theremaining silicon from layer 103, is the channel region 119, seen inFIG. 16.

Raised source and drain regions are then epitaxially grown to establisha shallow, highly doped source/drain tip (extension) that laterallyextends the distance under the gate edge to the channel region 119.Separate processing is used for the p-channel and n-channel transistorswith each of the source and drain regions being grown in differentprocessing, both with in-situ doping. This results in the source anddrain regions being highly doped, in one case with a p-type dopant, andin the other case with an n-type dopant.

In forming a PMOS transistor, the source and drain regions are raised asillustrated. They may be formed by selectively depositing epitaxialboron (B) doped silicon or SiGe with germanium concentrations up to 30%,as an example. Under the processing conditions of 100 sccm ofdichlorosilane (DCS), 20 slm H₂, 750-800° C., 20 Torr, 150-200 sccm HCl,a diborane (B₂H₆) flow of 150-200 sccm and a GeH₄ flow of 150-200 sccm,a highly doped SiGe film with a deposition rate of 20 nm/min, Bconcentration of 1E20 cm⁻³ and a germanium concentration of 20% isachieved. A low resistivity of 0.7-0.9 mOhm-cm resulting from the high Bconcentration in the film provides the benefit of high conductivity inthe tip source/drain regions and thereby reduced R_(external). SiGe inthe source/drain regions exerts compressive strain on the channel, whichin turn results in enhanced mobility and improved transistorperformance.

For an NMOS transistor, the source/drain regions are formed, forinstance, using in-situ phosphorous doped silicon deposited selectivelyunder processing conditions of 100 sccm of DCS, 25-50 sccm HCl, 200-300sccm of 1% PH₃ with a carrier H₂ gas flow of 20 slm at 750° C. and 20Torr. A phosphorous concentration of 2E20 cm⁻³ with a resistivity of0.4-0.6 mOhm-cm is achieved in the deposited film.

The resultant structure is shown in FIG. 17. Ion implantation of boronmay be used to more heavily dope the source and drain region beyond theedges of the gate structure shown in FIG. 17.

A dielectric layer 130 is now formed over the structure of FIG. 17. Thiscorresponds to the dielectric layers 40 and 400 in the priorembodiments. Again, this layer may be an ILD layer or a sacrificiallayer. An etchant is used to etch away the dummy gate, providing anopening 140 seen in FIG. 18. This exposes the underlying oxide layer110. The oxide layer 110 is removed with an ordinary etchant as shown inFIG. 19, thereby exposing the channel region 119.

Now, the channel region can be etched to reduce its cross section asshown in FIG. 20. A tetramethylammonium hydroxide (TMAH) or ammoniumhydroxide solution with an appropriate pH value is used to selectivelyetch the exposed n type or intrinsic silicon channel region 119 withoutaffecting the p+ source and drain regions. This etchant is highlyselective, and thus leaves in place the tips 150 of the source and drainregions 120 while the thinning of the channel region 119 occurs. Theboron-doped silicon has a sufficiently different lattice energy than thephosphorous- or arsenic-doped silicon, thereby allowing this selectiveetching to occur. In one process, this etching is done at a megasonicenergy level of between 600 and 1100 kHz. The tips 150 shown in FIG. 20,thus remain even though the channel region falls below the raised sourceand drain regions.

A high k dielectric gate layer 122 may next be conformally depositedusing, for instance, ALD. Following this, metal gate layer 124 isformed. The appropriate work function for the layer 124 is used asdiscussed above for the layer 61.

The structure of FIG. 21 is planarized with, for instance, CMP toprovide the structure of FIG. 22. In the finished device, the finalrecessed channel 119 thus has raised, source/drain extension regions.This allows the current from the channel inversion layer to spreadupward into these extension regions as shown by the lines 125. Incontrast in examining FIG. 1B at 16, the current can only spread outwardand downward, thereby resulting in higher series resistance.

The transistor fabricated as described above may be incorporated into anintegrated circuit, central processing unit, which in turn is part of acomputing device or system. FIG. 23 illustrates such a system 600 inaccordance with one embodiment. As illustrated, for the embodiment,system 600 includes computing device 602 for processing data. Computingdevice 602 may include a motherboard 604. Motherboard 604 may include inparticular a processor 606, and a networking interface 608 coupled to abus 610. More specifically, processor 606 may comprise the transistorsof FIG. 11 or 22, as examples, of the above-described transistor.

Depending on the applications, system 600 may include other components,including but are not limited to, volatile and non-volatile memory, agraphics processor, a digital signal processor, a crypto processor, achipset, mass storage (such as hard disk, compact disk (CD), digitalversatile disk (DVD) and so forth), and so forth.

In various embodiments, system 600 may be a personal digital assistant(PDA), a mobile phone, a tablet computing device, a laptop computingdevice, a desktop computing device, a set-top box, an entertainmentcontrol unit, a digital camera, a digital video recorder, a CD player, aDVD player, or other digital device of the like.

Thus, improved processing has been described that allows a channelregion to be controllably thinned, and that permits the source/drainregions to have less resistance.

1. A method for fabricating a transistor comprising: defining asemiconductor body; forming a dummy gate over the body to define achannel region in the body; implanting the body to form a source regionand drain region; surrounding the dummy gate with an insulatingmaterial; removing the dummy gate; etching the channel region of thebody; and forming a permanent gate on the etched channel region.
 2. Themethod defined by claim 1, wherein the etching of the channel region ofthe body reduces the cross-sectional area of the body in the channelregion without reducing the cross-sectional area of the source regionand drain region.
 3. The method defined by claim 1, wherein theimplanting the body comprises: using a first ion implant in alignmentwith the dummy gate to form a tip source region and a tip drain region;forming spacers on sides of the dummy gate; using a second ion implantin alignment with the spacers to form a main source region and a maindrain region.
 4. The method defined by claim 1, wherein the forming ofthe permanent gate comprises forming a high-k insulation over thechannel region and forming a metal gate on the insulation.
 5. The methoddefined by claim 4, wherein the spacers comprise a carbon doped nitride.6. The method defined by claim 4, wherein the metal gate has a workfunction between 3.9 and 5.2 eV.
 7. A method for fabricating atransistor comprising: defining a semiconductor body with a mask;forming a dummy gate over the body to define a channel region in thebody, leaving the mask in place on at least a portion of the bodycovered by the dummy gate; implanting the body to form a source anddrain region; surrounding the dummy gate with dielectric material;removing the dummy gate; etching the channel region of the body with anetchant which etches the body with the mask in place so as to reduce thewidth of the body disposed beneath the mask; removing the mask; andforming a permanent gate structure on the channel region.
 8. The methoddefined by claim 7, wherein the implanting of the body comprises: usinga first implant in alignment with the dummy gate to form tip source anddrain regions; forming spacers on sides of the dummy gate; and using asecond implant in alignment with the spacers to form main source anddrain regions.
 9. The method defined by claim 8, wherein the forming ofthe permanent gate structure comprises forming a high-k insulation overthe channel region and forming a metal gate on the insulation.
 10. Themethod defined by claim 9, wherein the metal gate has a work functionbetween 3.9 and 5.2 eV.
 11. The method defined by claim 9, wherein thespacers comprise a carbon doped nitride.
 12. A method for fabricating atransistor comprising: forming a dummy gate structure on an n-type orintrinsic semiconductor region; etching the semiconductor region so asto undercut the dummy gate structure to define a channel region; growingp type source and drain regions which extend into the undercut, adjacentthe channel region; removing at least part of the dummy gate structureso as to expose the channel region; etching the channel region with anetchant which discriminates between the p type source and drain regionsand the n type or intrinsic channel region so as to reduce thecross-sectional area of the channel region without substantiallyreducing the cross-sectional area of the source and drain regions; andforming a permanent gate structure.
 13. The method defined by claim 12,wherein the channel region is etched to a height which is below a heightof the source and drain regions.
 14. The method defined by claim 13,wherein the etching of the channel region employs a wet etchant.
 15. Themethod defined by claim 14, wherein the wet etchant comprisestetramethylammonium hydroxide or ammonium hydroxide solution.
 16. Themethod defined by claim 13, wherein the etching is done at an energylevel of between 600 and 1100 kHz.
 17. The method defined by claim 12,wherein the forming of the gate structure comprises forming a high-kmetal oxide dielectric and a metal gate.
 18. The method defined by claim17, wherein the metal gate has a work function of between 4.6 and 5.2eV.
 19. The method defined by claim 12, wherein the semiconductor regionis formed on a silicon-on-insulator substrate.
 20. The method defined byclaim 12, wherein the semiconductor region is formed on a delta-dopedsubstrate.